Acoustic wave device with suppressed higher order transverse modes

ABSTRACT

In an acoustic wave device, an unnecessary high-order transverse mode wave is suppressed. The acoustic wave device includes a piezoelectric substrate, at least one pair of interdigital transducer (IDT) electrodes formed on the piezoelectric substrate, and a dielectric film which covers at least a part of the piezoelectric substrate and the IDT electrodes. The IDT electrodes each have a plurality of electrode fingers interleaved with each other. An acoustic velocity of an acoustic wave in the area in which the electrode fingers are interleaved with each other is greater than an acoustic velocity of an acoustic wave in an edge area including end portions of the electrode fingers.

RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §120 as acontinuation of U.S. application Ser. No. 13/992,624, titled “ACOUSTICWAVE DEVICE WITH REDUCED HIGHER ORDER TRANSVERSE MODES,” filed on Jun.7, 2013, which is the U.S. National Phase under 35 U.S.C. §371 ofInternational Application No. PCT/JP2012/001495, filed on Mar. 5, 2012,which in turn claims the benefit of Japanese Application No.2011-068857, filed Mar. 25, 2011, the disclosures of which Applicationsare incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to acoustic wave devices used for bandpassfilters and the like.

BACKGROUND ART

In recent years, in the field of information communication devices suchas mobile phones, acoustic wave devices having a comb-shaped IDTelectrode formed on a surface of a piezoelectric substrate are used ascircuit elements such as resonators, filters, and the like. FIG. 26shows examples of such acoustic wave devices. In (a) of FIG. 26, a topview of an acoustic wave device 900 is shown. The acoustic wave device900 is formed by arranging two IDT electrodes 902 and two reflectors 903on a piezoelectric substrate 901. The IDT electrodes 902 each have a busbar 911 and a plurality of electrode fingers 912 that extends from thebus bar 911. The respective electrode fingers 912 of each of the IDTelectrodes 902 are arranged such that the electrode fingers 912 thereofare arranged interleaved with the respective electrode fingers 912 ofthe other IDT electrode 902. The reflectors 903 are arranged so as tointerpose the IDT electrodes 902 therebetween. In (b) of FIG. 26, a topview of another acoustic wave device 950 is shown. The acoustic wavedevice 950 is different from the acoustic wave device 900 in that dummyelectrode fingers 913 that extend, from the bus bar 911, alternatelywith the electrode fingers 912 are further provided, and in that theelectrode fingers 912 are arranged so as to face the dummy electrodefingers 913 of the other IDT electrode 902. In each of the acoustic wavedevices 900 and 950, a strip-shaped intersection area within the regionin which the electrode fingers 912 are arranged interleaved with eachother, excluding end portions of the respective electrode fingers 912,is used as a main propagation path of an acoustic wave.

Patent Literature 1 discloses an acoustic wave device 1000 which isdifferent from the acoustic wave device 900 in that a waveguide isformed by providing a coating film 1004 that serves as a piezoelectricbody or an insulator in the intersection area. FIG. 27 is a sectionalview of the acoustic wave device 1000 aligned along a single electrodefinger 912. In the acoustic wave device 1000, propagation of an acousticwave is concentrated to the coating film 1004, thereby suppressing ahigh-order transverse mode wave which is an unnecessary wave.

Patent Literature 2 discloses: an acoustic wave device 1100 which isdifferent from the acoustic wave device 900 in that, in each of twostrip-shaped edge areas adjacent to the intersection area, including endportions of the respective electrode fingers 912, the electrode fingers912 of the IDT electrode each have an increased width and an increasedfilm thickness; and an acoustic wave device 1150 which is different fromthe acoustic wave device 900 in that a dielectric film 1154 is providedon the IDT electrode only in the edge areas. In (a) of FIG. 28, a topview of the acoustic wave device 1100 is shown. In (b) of FIG. 28, asectional view of the acoustic wave device 1150 aligned along a singleelectrode finger 912 is shown. It is generally known that, in anacoustic wave device, acoustic velocities in edge areas and bar areasare decreased so as to be less than an acoustic velocity in anintersection area. Accordingly, in the intersection area that serves asa main propagation path, a fundamental transverse mode wave can beconfined and a high-order transverse mode wave which is an unnecessarywave can be suppressed. However, in Patent Literature 2, acousticvelocities in the edge areas are controlled by increasing end portionsof the electrode fingers of the IDT electrode in size and mass.

Patent Literature 3 discloses an acoustic wave device 1200 which isdifferent from the acoustic wave device 900 in that a dielectric film1204 covers the piezoelectric substrate 901, the IDT electrodes 902, andthe reflectors 903 such that the surface thereof is flattened. FIG. 29is a sectional view of the acoustic wave device 1200 aligned along asingle electrode finger 912. In the acoustic wave device 1200, thedielectric film 1204 covers the piezoelectric substrate 901, therebyreducing frequency characteristic change of the acoustic wave device1200 depending on temperature.

CITATION LIST Patent Literature

-   -   PTL 0: Japanese Patent No. 3201088    -   PTL 2: U.S. Pat. No. 7,576,471    -   PTL 3: PCT International Publication No. WO 2005/034347        (Japanese Application No. 2005-509259)

SUMMARY OF INVENTION Technical Problem

Conventionally, in an acoustic wave device, frequency characteristicchange depending on temperature may be stabilized by covering at leastan intersection area and edge areas with a dielectric film; however, noconsideration has been given to suppressing a high-order transverse modewave. Consequently, even in an acoustic wave device which is covered bya dielectric film for stabilizing the frequency characteristic changedepending on temperature, in order to control a high-order transversemode, shapes of end portions and the like of electrode fingers of an IDTelectrode need to be specially designed.

Therefore, an object of the present invention is, in an acoustic wavedevice in which a dielectric film covers at least an intersection areaand edge areas, to suppress high-order transverse mode wave which is anunnecessary wave.

Solution to Problem

An acoustic wave device of the present invention includes: apiezoelectric substrate; at least a pair of IDT electrodes formed on thepiezoelectric substrate; and a dielectric film which covers at least apart of the piezoelectric substrate and the IDT electrodes, and the pairof IDT electrodes each include a bus bar and a plurality of electrodefingers which extend from the bus bar, and the respective electrodefingers of one of the IDT electrodes are arranged interleaved with therespective electrode fingers of the other IDT electrode. The dielectricfilm covers at least a region in which the electrode fingers arearranged interleaved with each other. An acoustic velocity of anacoustic wave in an intersection area, within the region, which is aportion from ends of the electrode fingers to a predetermined length ormore inward from the ends, is greater than an acoustic velocity of anacoustic wave in an edge area which is a portion adjacent to theintersection area, including end portions of the electrode fingers.

Further, each of the pair of IDT electrodes may be connected to the busbar and include a plurality of dummy electrode fingers which are notarranged interleaved with the electrode fingers of the other IDTelectrode.

Either an area, adjacent to the edge area, from the ends of theelectrode fingers to the bus bar of the other IDT electrode, or an area,adjacent to the edge area, from the ends of the electrode fingers toends of the dummy electrode fingers which face the electrode fingers, isa gap area, the acoustic velocity in each of the gap areas is greaterthan the acoustic velocity in each of the edge areas.

A material of the piezoelectric substrate is lithium niobate, a materialof the dielectric film is silicon dioxide, and a film thickness of thedielectric film in the intersection area is thinner than a filmthickness of the dielectric film in the edge area.

Furthermore, the dielectric film may preferably have a laminationstructure including two kinds of silicon dioxide films or a laminationstructure including a silicon dioxide film and a dielectric film thatfurther covers the silicon dioxide film.

Alternatively, it is preferable that a first dielectric film whichcovers at least the area in which the electrode fingers are arrangedinterleaved with each other and a second dielectric film which is formedon the first dielectric film and further covers the intersection areaare included as the dielectric film, and an acoustic velocity of thesecond dielectric film is greater than an acoustic velocity of the firstdielectric film.

In this case, it is preferable that a material of the first dielectricfilm is silicon dioxide, and a material of the second dielectric film isany of silicon nitride, silicon oxynitride, and aluminium nitride.

Alternatively, it is preferable that a first dielectric film whichcovers at least the area in which the electrode fingers are arrangedinterleaved with each other; and a third dielectric film which is formedon the first dielectric film and further covers the edge areas areincluded as the dielectric film, and an acoustic velocity of the thirddielectric film is less than an acoustic velocity of the firstdielectric film.

In this case, it is preferable that a material of the first dielectricfilm is silicon dioxide and a material of the third dielectric film istantalum(V) oxide.

Further, it is preferable that the intersection area of the dielectricfilm is covered with a passivation film having a moisture resistancehigher than a moisture resistance of the dielectric film.

It is preferable that a region including the intersection area of thedielectric film is covered with a passivation film having a moistureresistance higher than a moisture resistance of the dielectric film, anda film thickness of the passivation film in the intersection area isthicker than a film thickness in the region other than the intersectionarea.

It should be noted that it is preferable that at least a part of anelectrode film thickness of the bus bar is thicker than an electrodefilm thickness of the electrode fingers.

Alternatively, the dielectric film may cover the dummy electrode fingersor the area in which the bus bar is arranged with a film thicknessthinner than a film thickness with which the dielectric film covers theintersection area.

Further, it is preferable that the piezoelectric substrate has a cutangle and a propagation angle of, when they are represented by the Eulerangle (phi, theta, psi) of right-handed orthogonal coordinates, thetabeing equal to or greater than 36 degrees but equal to or less than 41degrees.

Further, it is preferable that the IDT electrode has a laminationstructure including a first metal layer mainly composed any of platinum,tungsten, and molybdenum and a second metal layer laminated on the firstmetal layer having a higher conductivity than the first metal layer.

Further, the acoustic wave device may further includes two reflectorsformed on the piezoelectric substrate, and a plurality of pairs of theIDT electrodes may be formed between the two reflectors, and the tworeflectors and the plurality of pairs of the IDT electrodes may bearranged along a propagating direction of a main acoustic wave, therebyconstituting a longitudinally coupled filter.

Further, it is preferable that a tapered portion forms a portion of eachof ends of the intersection area and/or the edge areas of the dielectricfilm where a film thickness of the dielectric film changes, in a mannersuch that the film thickness of the dielectric film continuously changesat the tapered portion.

Advantageous Effects of Invention

According to the present invention, in an acoustic wave device in whicha dielectric film covers at least an intersection area and edge areas,it is possible to suppress a high-order transverse mode wave which is anunnecessary wave.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a top view and a sectional view of an acoustic wave deviceaccording to a first embodiment of the present invention.

FIG. 2 shows characteristics of the acoustic wave device according tothe first embodiment of the present invention.

FIG. 3 shows a modification of the acoustic wave device according to thefirst embodiment of the present invention.

FIG. 4 shows another modification of the acoustic wave device accordingto the first embodiment of the present invention.

FIG. 5 shows another modification of the acoustic wave device accordingto the first embodiment of the present invention.

FIG. 6 shows another modification of the acoustic wave device accordingto the first embodiment of the present invention.

FIG. 7 shows another modification of the acoustic wave device accordingto the first embodiment of the present invention.

FIG. 8 is a top view and a sectional view of an acoustic wave deviceaccording to a second embodiment of the present invention.

FIG. 9 shows characteristics of the acoustic wave device according tothe second embodiment of the present invention.

FIG. 10 shows a modification of the acoustic wave device according tothe second embodiment of the present invention.

FIG. 11 shows another modification of the acoustic wave device accordingto the second embodiment of the present invention.

FIG. 12 shows another modification of the acoustic wave device accordingto the second embodiment of the present invention.

FIG. 13 shows another modification of the acoustic wave device accordingto the second embodiment of the present invention.

FIG. 14 shows another modification of the acoustic wave device accordingto the second embodiment of the present invention.

FIG. 15 is a top view and a sectional view of an acoustic wave deviceaccording to a third embodiment of the present invention.

FIG. 16 shows a modification of the acoustic wave device according tothe third embodiment of the present invention.

FIG. 17 shows another modification of the acoustic wave device accordingto the third embodiment of the present invention.

FIG. 18 shows another modification of the acoustic wave device accordingto the third embodiment of the present invention.

FIG. 19 shows another modification of the acoustic wave device accordingto the third embodiment of the present invention.

FIG. 20 shows another modification of the acoustic wave device accordingto the third embodiment of the present invention.

FIG. 21 is a top view and a sectional view of an acoustic wave deviceaccording to a fourth embodiment of the present invention.

FIG. 22 shows a modification of the acoustic wave device according tothe embodiments of the present invention.

FIG. 23 shows another modification of the acoustic wave device accordingto the embodiments of the present invention.

FIG. 24 shows another modification of the acoustic wave device accordingto the embodiments of the present invention.

FIG. 25 shows another modification of the acoustic wave device accordingto the embodiments of the present invention.

FIG. 26 is a top view of a conventional acoustic wave device.

FIG. 27 is a sectional view of a conventional acoustic wave device.

FIG. 28 is a top view of a conventional acoustic wave device.

FIG. 29 is a sectional view of a conventional acoustic wave device.

DESCRIPTION OF EMBODIMENTS First Embodiment

Hereinafter, a first embodiment of the present invention will bedescribed. FIG. 1 is a see-through top view of an acoustic wave device100 according to the present embodiment and a sectional view of the sametaken along a line A-A′. The acoustic wave device 100 includes apiezoelectric substrate 101, and two IDT electrodes 102 and tworeflectors 103 arranged on the piezoelectric substrate 101. These arecovered by a dielectric film 104. The IDT electrodes 102 each have a busbar 111, and a plurality of electrode fingers 112 and a plurality ofdummy electrode fingers 113, which alternately extend from the bus bar111. The respective electrode fingers 112 from one side of the IDTelectrodes 102 are arranged interleaved with the respective electrodefingers 112 from the other side of the IDT electrode 102, in a mannersuch that ends of the electrode fingers 112 face the ends of the dummyelectrode fingers 113 from the other side of the IDT electrode 102. Thereflectors 103 are arranged so as to interpose the IDT electrodetherebetween. It is preferred if the piezoelectric substrate is lithiumniobate which has a cut angle and a propagation angle of, when they arerepresented by the Euler angle (phi,theta,psi) of right-handedorthogonal coordinates, theta being equal to or greater than 36 degreesbut equal to or less than 41 degrees. It should be noted that phi andpsi are each an arbitrary value between equal to or greater than −10degrees but equal to or less than 10 degrees. It is further preferred ifthe IDT electrodes 102 and the reflectors 103 each have a configurationincluding a lamination structure including a first metal layer mainlycomposed any of platinum, tungsten, and molybdenum and a second metallayer laminated on the first metal layer having a higher conductivitythan the first metal layer. With this, it is possible to reducecharacteristics loss and control electrode resistance of the acousticwave device 100. Further, an example of material for the dielectric filmis silicon dioxide (SiO₂).

In the acoustic wave device 100, a strip-shaped area within a region inwhich the respective electrode fingers 112 are arranged interleaved witheach other, excluding an area from the ends of the respective electrodefingers to a predetermined length is referred to as intersection area.Further, in the region in which the respective electrode fingers 112 arearranged interleaved with each other, two strip-shaped areas eachadjacent to the intersection area, including an area from the ends ofthe respective electrode fingers 112 to the predetermined length, arereferred to as edge areas. Still further, two strip-shaped areasadjacent to the respective edge areas, each including an area betweenthe ends of the respective electrode fingers 112 and the ends of therespective dummy electrode fingers 113 that face the correspondingelectrode fingers 112, are called gap areas. Yet further, twostrip-shaped areas adjacent to the respective gap areas, each includingan area between the ends of the respective dummy electrode fingers 113and the edges of the bus bar 111 on the side from which the electrodefingers extend, are referred to as dummy areas. The acoustic wave device100 uses a Rayleigh wave as a main acoustic wave and, the intersectionarea among the areas described above is used as a main propagation path.

In the acoustic wave device 100, a dielectric film is formed such that afilm thickness thereof in the intersection area is thinner than a filmthickness thereof in the edge areas. With this configuration, in theacoustic wave device 100, (acoustic velocity in the intersectionarea)>(acoustic velocity in the edge areas) is established. Further, inthe acoustic wave device 100, (acoustic velocity in the gapareas)>(acoustic velocity in the edge areas) is established.Consequently, a fundamental transverse mode wave can be confined in theintersection area and a high-order transverse mode wave which isspurious can be suppressed.

Further, in the acoustic wave device 100, by covering a part of the busbar areas with a dielectric film, a passivation effect on the bus barareas can be obtained. At this time, the relationship among the acousticvelocities is represented by (acoustic velocity in the intersectionarea)>(acoustic velocity in the bus bar areas), and (acoustic velocityin the edge areas)>(acoustic velocity in the bus bar areas).

FIG. 2 is a graph showing frequency characteristics of the admittance inthe acoustic wave device 100, in cases where: the film thickness of thedielectric film is 1300 nanometers on the IDT electrode; the filmthickness of the dielectric film is thinner in the intersection areathan in the edge areas by 300 nanometers; and the film thickness of thedielectric film is thinner in the intersection area than in the edgearea by 600 nanometers. Here, the pitch of the IDT electrodes in theacoustic wave device 100 is 2 micrometers and a half of a wave lengthlambda of the main acoustic wave. Further, in the acoustic wave device100, when the film thickness of the dielectric is normalized by the wavelength lambda of the main acoustic wave, the film thickness thedielectric film of 1300 nanometers described above is 0.325 lambda, thefilm thickness of the dielectric film when it is thinner by 300nanometers described above is 0.25 lambda, and the film thickness of thedielectric film when it is thinner by 600 nanometers described above is17.5%. As a comparative example, FIG. 2 also shows a frequencycharacteristic when, in the acoustic wave device 100, the film thicknessof the dielectric film in the intersection area is not thinner as in theconventional acoustic wave device. As shown in FIG. 2, it is appreciatedthat the acoustic wave device 100 of the present embodiment suppresses atransverse mode spurious when compared to the comparative example. Itshould be noted that, in the above description, the wave length lambdaof the main acoustic wave is 4 micrometers; however, when another wavelength is wave length lambda of the main acoustic wave, the filmthickness of the dielectric film may be a film thickness normalized bythe wave length lambda as described above.

Further, FIG. 3 shows a modification of the present embodiment. FIG. 3is a sectional view showing a section of an acoustic wave device 300according to the present embodiment in the same manner as the acousticwave device 100 shown in FIG. 1. The acoustic wave device 300 isdifferent from the acoustic wave device 100 in that the film thicknessof the dielectric film 104 in the gap area, or in and outside the dummyarea thinner than the film thickness of the dielectric film 104 in theedge area. Accordingly, an acoustic wave can be suppressed from leakingfrom the intersection area which is a main propagation path, therebyfurther enhancing confinement of the acoustic wave in the intersectionarea.

Further, as shown in FIG. 4, by covering the dielectric film 104 by apassivation film 108 having a higher moisture resistance than thedielectric film 104, moisture resistance of the acoustic wave device 100can be improved. Examples of this passivation film are silicon nitride(SiN), silicon oxynitride (SiON), and aluminium nitride (AlN).

Still further, as shown in FIG. 5, at least a part of the electrode filmthickness of the bus bar 111 is made thicker than the electrode filmthickness of the electrode fingers 112, thereby suppressing electroderesistance of the IDT electrodes 102 and improving characteristics ofthe acoustic wave device 100. It should be noted that, instead of thebus bar 111, the film thickness of a wiring electrode (not shown)connected to a bus bar may be made thicker than the electrode fingers112 of the IDT electrodes 102. In this case, wiring resistance of theacoustic wave device 100 can be suppressed and insertion loss of theacoustic wave device 100 can be reduced.

As shown in FIG. 6, the dielectric film 104 may have a laminationstructure including silicon dioxide films 104 a, 104 b 1. In this case,the silicon dioxide film 104 a is laminated on the entirety of a topface of the IDT electrodes 102, and a silicon dioxide film 104 b islaminated on an area excluding the intersection area on the top face ofthe IDT electrodes 102, the area including the edge areas, the gapareas, and the dummy areas. Accordingly, yield in the manufacturingprocess of the acoustic wave device 100 will be increased. It should benoted that the silicon dioxide film 104 b may be a dielectric film madeof another material having a greater acoustic velocity than a silicondioxide film.

The acoustic wave device 100 in which the IDT electrodes 102 includesthe dummy electrode fingers 113 has been described above; however, asshown in FIG. 7, the IDT electrodes 102 may not include the dummyelectrode fingers 113. In this case, the IDT electrodes 102 have nodummy area and each gap area is an area between the ends of therespective electrode fingers 112 and the bus bar of the other IDTelectrode. In this case also, in the acoustic wave device 100, (acousticvelocity in the intersection area)>(acoustic velocity in the edgeareas), and (acoustic velocity in the gap areas)>(acoustic velocity inthe edge areas) are established. Consequently, a fundamental transversemode wave can be confined in the intersection area and a high-ordertransverse mode wave which is spurious can be suppressed.

Second Embodiment

Hereinafter, a second embodiment of the present invention will bedescribed. FIG. 8 is a see-through top view of an acoustic wave device400 according to the present embodiment and a sectional view of the sametaken along a line A-A′. The acoustic wave device 400 includes, as inthe same manner as the acoustic wave device 100 of the first embodiment,the piezoelectric substrate 101 and two IDT electrodes 102 and tworeflectors 103 arranged on the piezoelectric substrate 101. However, theacoustic wave device 400 is different from the acoustic wave device 100in that these components are covered with a first dielectric film 404and further covered with a second dielectric film 405.

In the acoustic wave device 400, the first dielectric film 404 coversthe piezoelectric substrate 101, the IDT electrodes 102, and thereflectors 103 so as to form a flat surface. Further, the seconddielectric film 405 covers the intersection area on the first dielectricfilm 404 with a constant film thickness. An example of material of thefirst dielectric film 404 is silicon dioxide (SiO₂). Examples ofmaterial of the second dielectric film 405 are silicon nitride (SiN),silicon oxynitride (SiON), aluminium oxide (Al₂O₃), aluminium nitride(AlN), and the like. These materials are selected so that the seconddielectric film 405 has an acoustic velocity greater than an acousticvelocity of the first dielectric film 404.

In the acoustic wave device 400, (acoustic velocity in the intersectionarea)>(acoustic velocity in the edge areas) is established due to aninfluence of the second dielectric film 405 that covers the intersectionarea. Further, in the acoustic wave device 400, (acoustic velocity inthe gap areas)>(acoustic velocity in the edge areas) is established.Consequently, a fundamental transverse mode wave can be confined in theintersection area and a high-order transverse mode wave which isspurious can be suppressed.

Still further, in the acoustic wave device 400, by covering a part ofthe bus bar area with the dielectric film, a passivation effect on thebus bar area can be obtained. At this time, the relationship among theacoustic velocities is represented by (acoustic velocity in theintersection area)>(acoustic velocity in the bus bar area), and(acoustic velocity in the edge areas)>(acoustic velocity in the bus bararea).

FIG. 9 is a graph showing a frequency characteristic of the admittance,when, in the acoustic wave device 400, the second dielectric film 405has a film thickness of 30 nanometers. Here, the pitch of the IDTelectrodes 102 in the acoustic wave device 400 is 2 micrometers, and thefilm thickness of the dielectric film is 0.015 lambda when normalized bythe wave length lambda of the main acoustic wave. As a comparativeexample, FIG. 9 also shows a frequency characteristic when, in theacoustic wave device 400, the second dielectric film 405 is notincluded. As shown in FIG. 9, it is appreciated that the acoustic wavedevice 400 of the present embodiment controls a transverse mode spuriouswhen compared to the comparative example. It should be noted that, inthe above description, the wave length lambda of the main acoustic waveis 4 micrometers; however, when another wave length is wave lengthlambda of the main acoustic wave, the film thickness of the dielectricfilm may be a film thickness normalized by the wave length lambda asdescribed above.

FIG. 10 shows a modification of the present embodiment. FIG. 10 shows asection of an acoustic wave device 600 according to the presentembodiment, in the same manner as the sectional view of the acousticwave device 100 shown in FIG. 1. The acoustic wave device 600 isdifferent from the acoustic wave device 400 in that the film thicknessof the first dielectric film 404 in the gap area, or in and outside thedummy area is thinner than the film thickness of the first dielectricfilm 404 in the edge area. Accordingly, an acoustic wave can besuppressed from leaking from the intersection area which is a mainpropagation path, thereby further enhancing confinement of the acousticwave in the intersection area.

It should be noted that the second dielectric film 405 is preferablyprovided on the first dielectric film 404 in the gap area or the dummyarea, in addition to the intersection area. With this, an acoustic wavecan be further suppressed from leaking from the intersection area whichis the main propagation path, thereby further enhancing the confinementof the acoustic wave in the intersection area.

Further, as shown in FIG. 11, by covering the first dielectric film 404by a passivation film 408 having a higher moisture resistance than thefirst dielectric film 404, moisture resistance of the acoustic wavedevice 400 can be improved. Examples of this passivation film aresilicon nitride (SiN), silicon oxynitride (SiON), and aluminium nitride(AlN).

Furthermore, as shown in FIG. 12, the passivation film 408 may beprovided between the first dielectric film 404 and the second dielectricfilm 405.

Further, as shown in FIG. 13, at least a part of the electrode filmthickness of the bus bar 111 is made thicker than the electrode filmthickness of the electrode fingers 112, thereby suppressing electroderesistance of the IDT electrodes 102 and improving characteristics ofthe acoustic wave device 400. It should be noted that, instead of thebus bar 111, the film thickness of a wiring electrode (not shown)connected to a bus bar may be made thicker than the electrode fingers112 of the IDT electrodes 102. In this case, wiring resistance of theacoustic wave device 400 can be suppressed and insertion loss of theacoustic wave device 400 can be reduced.

The acoustic wave device 400 in which the IDT electrodes 102 includesthe dummy electrode fingers 113 has been described above; however, asshown in FIG. 14, the IDT electrodes 102 may not include the dummyelectrode fingers 113. In this case, the IDT electrodes 102 have nodummy area and each gap area is an area between the ends the respectiveelectrode fingers 112 and the bus bar of the other IDT electrode. Inthis case also, in the acoustic wave device 400, (acoustic velocity inthe intersection area)>(acoustic velocity in the edge areas), and(acoustic velocity in the gap areas)>(acoustic velocity in the edgeareas) are established. Consequently, a fundamental transverse mode wavecan be confined in the intersection area and a high-order transversemode wave which is spurious can be suppressed.

Third Embodiment

Hereinafter, a third embodiment of the present invention will bedescribed. FIG. 15 is a see-through top view of an acoustic wave device700 according to the present embodiment and a sectional view of the sametaken along a line A-A′. The acoustic wave device 700 includes, as inthe same manner as the acoustic wave device 100 of the first embodiment,the piezoelectric substrate 101 and two IDT electrodes 102 and tworeflectors 103 arranged on the piezoelectric substrate 101. However, theacoustic wave device 700 is different from the acoustic wave device 100in that these components are covered with a first dielectric film 704and further covered with a third dielectric film 705.

The acoustic wave device 700 is different from the acoustic wave device400 of the second embodiment in that the third dielectric film 705covers the edge areas on the first dielectric film 704 with a constantfilm thickness. An example of material of the first dielectric film 704is silicon dioxide (SiO₂). Example of material of the third dielectricfilm 705 is tantalum(V) oxide (Ta₂O₅). These material are selected sothat the third dielectric film 705 has an acoustic velocity is less thanan acoustic velocity of first dielectric film 704.

In the acoustic wave device 700, (acoustic velocity in the intersectionarea)>(acoustic velocity in the edge areas) is established due to aninfluence of the third dielectric film that covers the edge areas.Further, in the acoustic wave device 700, (acoustic velocity in the gapareas)>(acoustic velocity in the edge areas) is established.Consequently, in the same manner as the first and second acoustic wavedevice 100 and 400, a high-order transverse mode wave which is spuriouscan be suppressed.

Still further, in the acoustic wave device 700, by covering a part ofthe bus bar area with the dielectric film, a passivation effect on thebus bar area can be obtained. At this time, the relationship among theacoustic velocities is represented by (acoustic velocity in theintersection area)>(acoustic velocity in the bus bar area), and(acoustic velocity in the edge areas)>(acoustic velocity in the bus bararea).

FIG. 16 shows a modification of the present embodiment. FIG. 16 shows asection of an acoustic wave device 800 according to the presentembodiment, in the same manner as the sectional view of the acousticwave device 100 shown in FIG. 1. The acoustic wave device 800 isdifferent from the acoustic wave device 700 in that the film thicknessof the first dielectric film 704 in the gap area, or in and outside thedummy area is thinner than the film thickness of the first dielectricfilm 704 in the edge area. Accordingly, an acoustic wave can besuppressed from leaking from the intersection area which is a mainpropagation path, thereby further enhancing confinement of the acousticwave in the intersection area.

In the above embodiments, rotated Y cut lithium niobate is used as thepiezoelectric substrate 101 and silicon dioxide is as dielectric films;however, the scope of the present invention can be also appropriatelyapplied to materials different from piezoelectric substrate anddielectric film. Further, the present invention can be also applied to acase where the IDT electrodes 902 does not include dummy electrodefingers in the same manner as the acoustic wave device 900 shown in FIG.9.

Further, as shown in FIG. 17, by covering the first dielectric film 704by a passivation film 708 having a higher moisture resistance than thefirst dielectric film 704, moisture resistance of the acoustic wavedevice 700 can be improved. Examples of this passivation film aresilicon nitride (SiN), silicon oxynitride (SiON), and aluminium nitride(AlN).

Furthermore, as shown in FIG. 18, the passivation film 708 may beprovided between the first dielectric film 704 and the second dielectricfilm 705.

Further, as shown in FIG. 19, at least a part of the electrode filmthickness of the bus bar 111 is made thicker than the electrode filmthickness of the electrode fingers 112, thereby suppressing electroderesistance of the IDT electrodes 102 and improving characteristics ofthe acoustic wave device 700. It should be noted that, instead of thebus bar 111, the film thickness of a wiring electrode (not shown)connected to a bus bar may be made thicker than the electrode fingers112 of the IDT electrodes 102. In this case, wiring resistance of theacoustic wave device 700 can be suppressed and insertion loss of theacoustic wave device 700 can be reduced.

The acoustic wave device 700 in which the IDT electrodes 102 includesthe dummy electrode fingers 113 has been described above; however, asshown in FIG. 20, the IDT electrodes 102 may not include the dummyelectrode fingers 113. In this case, the IDT electrodes 102 have nodummy area and each gap area is an area between the ends of therespective electrode fingers 112 and the bus bar of the other IDTelectrode. In this case also, in the acoustic wave device 700, (acousticvelocity in the intersection area)>(acoustic velocity in the edgeareas), and (acoustic velocity in the gap areas)>(acoustic velocity inthe edge areas) are established. Consequently, a fundamental transversemode wave can be confined in the intersection area and a high-ordertransverse mode wave which is spurious can be suppressed.

Fourth Embodiment

Hereinafter, a fourth embodiment of the present invention will bedescribed. FIG. 21 is a see-through top view of an acoustic wave device800 according to the present embodiment and a sectional view of the sametaken along a line A-A′. The acoustic wave device 800 is different fromthe acoustic wave device 100 in that, on the piezoelectric substrate101, a plurality of pairs of IDT electrodes 802 a, 802 b, 802 c areprovided between two reflectors 803 in the propagating direction of amain acoustic wave, thereby constituting a longitudinally coupledfilter. It should be noted that, FIG. 21, the acoustic wave device 800shown in FIG. 21 includes three pairs of IDT electrodes; however, theacoustic wave device 800 may include two, four, or more pairs of IDTelectrodes.

Also in the acoustic wave device 800, (acoustic velocity in theintersection area)>(acoustic velocity in the edge areas) is established.Further, in the acoustic wave device 800, (acoustic velocity in the gapareas)>(acoustic velocity in the edge areas) is established.Consequently, a high-order transverse mode wave which is spurious can besuppressed in the in the same manner as in the acoustic wave device 100.

Still further, in the acoustic wave device 00, by covering a part of thebus bar area with the dielectric film, a passivation effect on the busbar area can be obtained. At this time, the relationship among theacoustic velocities is represented by (acoustic velocity in theintersection area)>(acoustic velocity in the bus bar area), and(acoustic velocity in the edge areas)>(acoustic velocity in the bus bararea).

(Modification 1)

In each of the above embodiments, it is preferable that tapered portions1201 are provided at an end of each of the intersection area which isthe main propagation path of an acoustic wave and the edge areas, ateach end the film thickness of the dielectric film changes.Consequently, change of the film thickness becomes less sudden with thefilm thickness continuously changing. As an example, in (a) of FIG. 22,a sectional view of the acoustic wave device 100 according to the firstembodiment shown in FIG. 1, in which the tapered portions 1201 areprovided, on the dielectric film 104, at boundaries between theintersection area and each of the edge areas, is shown. As anotherexample, in (b) of FIG. 22, a sectional view of the acoustic wave device300 according to the first embodiment shown in FIG. 3, in which thetapered portions 1201 are provided, on the dielectric film 104, atboundaries between the intersection area and each of the edge areas andboundaries between the edge areas and the gap areas, is shown. As stillanother example, FIG. 23 shows a sectional view of the acoustic wavedevice 600 according to the second embodiment shown in FIG. 11, in whichthe tapered portions 1201 are provided, on the second dielectric film405 and the passivation film 408, at boundaries between the intersectionarea and each of the edge areas. As in these examples, when the filmthickness of the dielectric films is continuously changed by the taperedportions 1201, sudden change of the acoustic velocity of an acousticwave which propagates through the boundaries can be suppressed whencompared to when the film thickness is changed suddenly, therebyreducing undesired spurious waves.

(Modification 2)

In each of the above embodiments, when providing a passivation film, thepassivation film may be provided only in the intersection area. FIG. 24is sectional views of acoustic wave devices 110, 120, and 130 which aredifferent from the acoustic wave device 100 according to the firstembodiment in that the passivation film 108 covers only the intersectionarea of the dielectric film 104. In the acoustic wave device 110 shownin (a) of FIG. 24, the passivation film 108 has a thickness which isthinner than a difference between the film thickness in the intersectionarea and the film thickness in the edge area of the dielectric film 104.In the acoustic wave device 120 shown in (b) of FIG. 24, the passivationfilm 108 has a thickness which is equal to a difference between the filmthickness in the intersection area and the film thickness in the edgearea of the dielectric film 104. In the acoustic wave device 130 shownin (c) of FIG. 24, the passivation film 108 has a thickness which isthicker than a difference between the film thickness in the intersectionarea and the film thickness in the edge area of the dielectric film 104.

Further, in each of the above embodiments, a passivation film may beprovided in the intersection area and the other areas, and the filmthickness of the passivation film in the intersection area may bethicker than the film thickness in the other areas. (a) of FIG. 25 and(b) of FIG. 25 are sectional views of acoustic wave devices 140, 150which are different from the acoustic wave device 100 according to thefirst embodiment in that the passivation film 108 covers the entirety ofthe dielectric film 104, and has a thicker film thickness in theintersection area than in the other areas. In the acoustic wave device140, the passivation film 108 has a flat surface. In the acoustic wavedevice 150, the passivation film 108 has a surface which protrudes at aportion thereof covering the intersection area.

As in the acoustic wave devices 110 to 150, the passivation film 108having a greater acoustic velocity than that of the dielectric film 104covers only the intersection area or a region including the intersectionarea such that the film thickness is thicker in the intersection areathan in the region other than the intersection area, thereby increasinga difference between the acoustic velocity in the intersection area andthe acoustic velocity in the other areas. Consequently, an effect ofconfining a fundamental transverse mode wave in the intersection areacan be enhanced. Furthermore, even when the dielectric film 104 and theelectrode fingers 112 have different thicknesses, by providing thepassivation film 108 with an appropriate thickness, an acoustic velocitycan be easily adjusted finely and a predetermined frequencycharacteristic can be obtained. The present modification is alsoapplicable to the acoustic wave devices according to the embodimentsother than the acoustic wave device 100.

Further, (c) of FIG. 25 is a sectional view of an acoustic wave device160 which is different from the acoustic wave device 100 according tothe first embodiment in that the dielectric film 104 has an eventhickness and that the passivation film 108 covers the entirety of thedielectric film 104 and has a thicker film thickness in the intersectionarea than in the other areas. If the acoustic velocity in theintersection area can be greater and a difference between the acousticvelocity in the intersection area and the acoustic velocity in the edgearea can be sufficiently large owing to the difference of the filmthickness of the passivation film 108, the dielectric film 104 may havean even film thickness in this manner.

Further, in each of the above modifications, the passivation film 108may have a tapered shape such that the film thickness thereofcontinuously changes at the end of the intersection area. Accordingly,undesired spurious waves can be reduced.

INDUSTRIAL APPLICABILITY

The present invention is useful for application to a surface acousticwave device used for an information communication device, and the like.

REFERENCE SIGNS LIST

-   100, 110, 120, 130, 140, 150, 160, 300, 400, 600, 700, 800, 900,    1000, 1100, 1150 acoustic wave device-   101, 901 piezoelectric substrate-   102, 902 IDT electrode-   103, 903 reflector-   104, 104 a, 104 b, 404, 405, 704, 705, 1154, 1204 dielectric film-   108, 408, 708 passivation film-   111, 911 bus bar-   112, 912 electrode finger-   113, 913 dummy electrode finger-   1004 coating film-   1201 tapered portion

The invention claimed is:
 1. An acoustic wave device comprising: apiezoelectric substrate; a pair of interdigital transducer (IDT)electrodes formed on the piezoelectric substrate, a first IDT electrodeof the pair of IDT electrodes including a first plurality of electrodefingers, a second IDT electrode of the pair of IDT electrodes includinga second plurality of electrode fingers, the first plurality ofelectrode fingers interleaved with the second plurality of electrodefingers; and a dielectric film covering at least a part of thepiezoelectric substrate and the pair of IDT electrodes, the dielectricfilm covering an intersection area in which the first plurality ofelectrode fingers and the second plurality of electrode fingers areinterleaved with each other, edge areas adjacent to the intersectionarea, and gap areas adjacent to the intersection areas, an acousticvelocity of an acoustic wave in the intersection area being greater thanthe acoustic velocity of the acoustic wave in the edge areas, thedielectric film including a first area covering the intersection areaand having a first thickness, and a second area covering the edge areasand the gap areas and having a second thickness, the second thicknessbeing greater than the first thickness.
 2. The acoustic wave device ofclaim 1 further comprising a passivation film formed on the dielectricfilm only in the intersection area.
 3. The acoustic wave device of claim2 wherein the passivation film has a thickness that is less than adifference between the first thickness and the second thickness.
 4. Theacoustic wave device of claim 2 wherein the passivation film has athickness that is the same as a difference between the first thicknessand the second thickness.
 5. The acoustic wave device of claim 2 whereinthe passivation film has a thickness that is greater than a differencebetween the first thickness and the second thickness.
 6. The acousticwave device of claim 1 further comprising a passivation film formed onthe dielectric film, the passivation film having a thickness in theintersection area greater than a thickness in the edge areas.
 7. Theacoustic wave device of claim 6 wherein the passivation film has a flatupper surface.
 8. The acoustic wave device of claim 6 wherein thepassivation film has an upper surface that protrudes at a portionthereof coving the intersection area.
 9. The acoustic wave device ofclaim 1 wherein the dielectric film includes a first dielectric film anda second dielectric film covering the first dielectric film in an areaexcluding the intersection area, the area including the edge areas andthe gap areas.
 10. An acoustic wave device comprising: a piezoelectricsubstrate; a pair of interdigital transducer (IDT) electrodes formed onthe piezoelectric substrate, a first IDT electrode of the pair of IDTelectrodes including a first plurality of electrode fingers, a secondIDT electrode of the pair of IDT electrodes including a second pluralityof electrode fingers, the first plurality of electrode fingersinterleaved with the second plurality of electrode fingers; and adielectric film covering at least a part of the piezoelectric substrateand the pair of IDT electrodes, the dielectric film covering anintersection area in which the first plurality of electrode fingers andthe second plurality of electrode fingers are interleaved with eachother, edge areas adjacent to the intersection area, and gap areasadjacent to the intersection areas, an acoustic velocity of an acousticwave in the intersection area being greater than the acoustic velocityof the acoustic wave in the edge areas, the dielectric film including afirst area covering the intersection area and having a first thickness,a second area covering the edge areas and having a second thickness, anda third area covering the gap areas and having a third thickness, thesecond thickness being greater than the first thickness, the firstthickness being greater than the third thickness.
 11. The acoustic wavedevice of claim 10 wherein the dielectric film in the edge areasincludes a first dielectric film and a second dielectric film formed onthe first dielectric film.
 12. The acoustic wave device of claim 11wherein the acoustic velocity of the acoustic wave in the seconddielectric film is less than the acoustic velocity of the acoustic wavein the first dielectric film.
 13. An acoustic wave device comprising: apiezoelectric substrate; a pair of interdigital transducer (IDT)electrodes formed on the piezoelectric substrate, a first IDT electrodeof the pair of IDT electrodes including a first plurality of electrodefingers, a second IDT electrode of the pair of IDT electrodes includinga second plurality of electrode fingers, the first plurality ofelectrode fingers interleaved with the second plurality of electrodefingers; a first dielectric film covering at least a part of thepiezoelectric substrate and the pair of IDT electrodes, the firstdielectric film covering an intersection area in which the firstplurality of electrode fingers and the second plurality of electrodefingers are interleaved with each other, edge areas adjacent to theintersection area, and gap areas adjacent to the intersection areas, anacoustic velocity of an acoustic wave in the intersection area beinggreater than the acoustic velocity of the acoustic wave in the edgeareas, the first dielectric film including a first area covering theintersection area and the edge areas and having a first thickness and asecond area covering the gap areas and having a second thickness, thefirst thickness being greater than the second thickness; and a seconddielectric film covering the first dielectric film in the intersectionarea.
 14. The acoustic wave device of claim 13 wherein the seconddielectric film does not cover the edge areas.
 15. The acoustic wavedevice of claim 13 wherein the acoustic velocity of the acoustic wave inthe second dielectric film is greater than the acoustic velocity of theacoustic wave in the first dielectric film.
 16. An acoustic wave devicecomprising: a piezoelectric substrate; a pair of interdigital transducer(IDT) electrodes formed on the piezoelectric substrate, a first IDTelectrode of the pair of IDT electrodes including a first plurality ofelectrode fingers, a second IDT electrode of the pair of IDT electrodesincluding a second plurality of electrode fingers, the first pluralityof electrode fingers interleaved with the second plurality of electrodefingers; a first dielectric film covering at least a part of thepiezoelectric substrate and the pair of IDT electrodes, the firstdielectric film covering an intersection area in which the firstplurality of electrode fingers and the second plurality of electrodefingers are interleaved with each other, edge areas adjacent to theintersection area, gap areas adjacent to the intersection areas, anddummy areas adjacent to the gap areas, an acoustic velocity of anacoustic wave in the intersection area being greater than the acousticvelocity of the acoustic wave in the edge areas, the first dielectricfilm having a same thickness in the intersection area, the gap areas,and the dummy areas; a passivation film covering the first dielectricfilm the intersection area, the gap areas, and the dummy areas; and asecond dielectric film covering the passivation film in the intersectionarea.
 17. The acoustic wave device of claim 16 wherein the passivationfilm has a higher moisture resistance than the first dielectric film.18. The acoustic wave device of claim 16 wherein the acoustic velocityof an acoustic wave in the second dielectric film is greater than theacoustic velocity of the acoustic wave in the first dielectric film. 19.An acoustic wave device comprising: a piezoelectric substrate; a pair ofinterdigital transducer (IDT) electrodes formed on the piezoelectricsubstrate, a first IDT electrode of the pair of IDT electrodes includinga first plurality of electrode fingers, a second IDT electrode of thepair of IDT electrodes including a second plurality of electrodefingers, the first plurality of electrode fingers interleaved with thesecond plurality of electrode fingers; a first dielectric film coveringat least a part of the piezoelectric substrate and the pair of IDTelectrodes, the first dielectric film covering an intersection area inwhich the first plurality of electrode fingers and the second pluralityof electrode fingers are interleaved with each other, edge areasadjacent to the intersection area, and gap areas adjacent to theintersection areas, an acoustic velocity of an acoustic wave in theintersection area being greater than the acoustic velocity of theacoustic wave in the edge areas, the first dielectric film having firstthickness in the intersection area and the edge areas, and a secondthickness in the gap areas, the first thickness being greater than thesecond thickness; and a second dielectric film covering the firstdielectric film in the intersection area.
 20. The acoustic wave deviceof claim 19 wherein the acoustic velocity of the acoustic wave in thesecond dielectric film is greater than the acoustic velocity of theacoustic wave in the first dielectric film.